This invention relates to electrolytic cell separators and more particularly, to methods of making separators which will serve as substitutes for asbestos diaphragms especially in cells used for the electrolysis of alkali-metal halide brines. Still more particularly, it relates to a post-manufacturing process for treating polymeric electrolytic cell separators, usually fluoropolymer-containing, which are inherently hydrophobic, and not wettable by the cell fluids.
Chlorine is produced almost entirely by electrolytic methods, primarily from aqueous solutions of alkali metal chloride. In the electrolysis of such solutions or brines chlorine is generated at the anode and alkali metal hydroxide, such as sodium or potassium hydroxide is produced at the cathode, together with hydrogen. Because the anode and cathode products should be kept apart to prevent reactions between them many cell designs have been developed to accomplish such separation. The designs have generally utilized either a diaphragm or a mercury intermediate electrode to separate the anolyte and catholyte.
In diaphragm cells brine is fed continuously into the cell and flows from the anode compartment through an asbestos diaphragm to the cathode compartment, which contains, for example, an iron cathode. To minimize back-diffusion and migration, the flow rate is maintained so that only a part of the salt present is electrolyzed. The hydrogen ions form hydrogen gas at the cathode, leaving hydroxyl ions in the cathode compartment. The catholyte solution, which contains sodium hydroxide and unchanged sodium chloride, is subsequently evaporated to obtain the hydroxide. In the course of such evaporation much of the sodium chloride precipitates and is separated, dissolved and sent back to the electrolytic cell, often as an aqueous solution or brine feed to the anolyte compartment. Thus, the function of the diaphragm is to maintain a desirably high concentration of alkali in the catholyte, to minimize the diffusional migration of hydroxyl ions into the anolyte and to maintain separation of chlorine from hydrogen and alkali metal hydroxide. The diaphragm should also have minimal electrical resistance to lower power consumption during electrolysis.
It has been customary to use asbestos deposited material as the diaphragm of choice, however, asbestos has not been entirely satisfactory. Asbestos diaphragms have a relatively short service life and asbestos has become a suspect health hazard. Consequently, synthetic substitutes for asbestos have been developed from fluoropolymers, such as polytetrafluoroethylene. A few of such fluoropolymer-based diaphragms are described in U.S. Pat. No. 3,890,417, U.S. Pat. No. 3,281,511 and U.S. Pat. No. 3,556,161.
Although fluoropolymer diaphragms have better service life expectancy and do not present the same potential health hazards as asbestos they nevertheless have the drawback of being hydrophobic and are not wettable by cell liquor. Wettability is troublesome in that it is difficult to achieve the desired flow characteristics of the electrolyte through the diaphragm if it is hydrophobic. Furthermore, if the diaphragm dewets while operating the cell will become inoperative for all practical purposes.
Heretofore, others have employed surface active agents and numerous other additives to polymeric diaphragms in an effort to solve the wettability problem. However, the results in many instances were less than satisfactory. For example U.S. Pat. No. 4,126,535 suggests the addition of fluorinated surfactants to the anolyte liquor of the cell in order to initiate flow through the diaphragm. This in-situ method of wetting polymeric diaphragams in the cell with surface active agents causes operational difficulties because internal cell components e.g. electrodes become coated with the surfactant. Furthermore, surfactants in many instances are foaming agents and their accumulation in the cell causes foam to collect in the gas headers and caustic collectors which become clogged. As a result, before start-up the cell must be drained and flushed with large volumes of water.
In U.S. Pat. No. 4,170,540 and continuation-in-part Ser. No. 064,616, filed Aug. 7, 1979 two of the three inventors are also co-inventors of this application. Such disclosures are not admitted to be prior art against the present application, but are referred to herein as being of interest. Both disclosures teach fluorosurfactants as lubricants used as additives during the process of making microporous PTFE diaphragms. After milling, the diaphragm sheets are dried to remove the volatile components of the lubricant additives, followed by high temperature sintering usually above the crystalline melting point of the polymer. Elevated temperatures for drying and sintering in combination with the use of acid to leach the pore former from the diaphragm eliminates most of the desired wetting properties which may have resulted from the use of surfactants during the manufacturing phase.
U.S. Pat. Nos. 3,930,886; 4,089,758; 4,126,536 and 4,153,530 disclose the addition of hydrophilic filler materials to the separator, including titanium dioxide, silicon dioxide, barium sulfate, potassium titanate and zirconium oxide. The addition of special fillers to the separator as wetting agents during the manufacturing phase has not given consistent, uniform results.
U.S. Pat. No. 4,012,541 discloses the use of an acetal-type non-ionic surfactant for wetting hydrophobic polymer diaphragms. Although the final results appear satisfactory the diaphragm is stored indefinitely in the surfactant to avoid drying out and is removed from the solution just prior to installation.
U.S. Pat. No. 4,125,451 discloses chlor-alkali cell diaphragms made from fluorinated hydrocarbon polymeric fibers which are first dispersed in an aqueous-acetone medium with surface active agents to form a slurry. The suspended fibers are deposited directly onto a cathode screen in the form of a porous network of fibers without the need for special bonding. The surfactants used to suspend the fluoropolymer fibers are either anionic or nonionic types which may be either non-fluorinated or fluorinated, including those available under such trademarks as FLUORAD FC-126 or FC-170 and ZONYL FSN, FSA or FSP fluorosurfactant. Apparently because of the diaphragm's high porosity the caustic produced was only 98 gpl at 81% current efficiency. Under such circumstances with such a highly porous diaphragm where the caustic concentration cannot build-up to commercially acceptable levels wettability is not viewed as a significant factor influencing diaphragm performance.
Accordingly, it has been discovered that polymeric based diaphragms can now be made to provide commercially acceptable performance characteristics which are at least equivalent in performance to asbestos-type diaphragms. Fluoropolymer diaphragms may be further treated after manufacturing to achieve high current efficiency, e.g. current efficiencies of at least 85% when measured in a chloralkali cell at a sodium hydroxide concentration of 150 gpl. The improved process imparts "permanent wettability" properties to a substantially hydrophobic material having hollow micropores particularly in diaphragms subjected to elevated temperatures for prolonged time periods during manufacturing. The application of a film of fluorinated surface active agent on the internal and external surfaces of the diaphragm imparts permanent wettability properties which only needs exposure to heated water or other aqueous solutions for activation prior to cell assembly or cell start-up. For purposes of the present invention "permanent wettability" is intended to mean a separator after being fitted into a chlor-alkali cell will remain stable and not lose its tendency to be wetted by the contents of the cell after being activated such that the surfaces do not become completely dry. It has been discovered that as long as pore wall surfaces remain moist either by direct contact with cell fluids or internal cell humidity wettability properties will not be lost. Unlike other processes, treated diaphragms according to the present invention may be dried for storage and shipping without losing wettability properties, and further treatment with chemical agents or additives can be eliminated.
Therefore, it is a principal object of the present invention to provide an improved method for imparting wettability properties to inherently hydrophobic microporous electrolytic cell separators.
It is further object of the present invention to provide a means for imparting wettability properties to fluoropolymer based microporous separators without requiring concomitant use in electrolytic cells.
A still further object of the present invention is to prepare fluoropolymer microporous diaphgrams which will provide performance characteristics at least equivalent to asbestos diaphragms which will not dewet while in serivce.
These and other objects, features and advantages will become apparent to those skilled in the art after a reading of the following more detailed description.